Micro-LED pick and place using metallic gallium

ABSTRACT

An LED die containing a gallium semiconductor layer is placed on a target substrate using a pick-up tool (PUT) attached to the LED die using metallic gallium. As a result of a laser lift-of (LLO) process to separate the gallium semiconductor layer from a substrate layer on which the gallium semiconductor layer is formed, a layer of gallium metal is formed on a surface of the LED die. The gallium layer is melted to form liquid gallium. A head of the PUT is contacted with the liquid gallium, whereupon the LED die is cooled such that the liquid gallium solidifies, attaching the LED die to the PUT. The PUT picks up and places the LED die at a desired location on a target substrate. The LED die can be heated to melt the gallium layer, allowing the PUT to be detached.

BACKGROUND

The present disclosure relates to an assembly process for use in, butnot exclusively limited to, pick and place of very small displayelement(s) that need to be transferred from an initial substrate to areceiving substrate using a pick-up and transfer process.

To populate a display with very small light emitting diodes (LEDs), suchas micro-LEDs, there may be a need to transfer the LEDs from the nativesubstrate on which they have been manufactured to a target substratethat forms part of a display, or “display substrate.” Such smallsemiconductor devices may be assembled with a defined separationdistance between them or closely packed together on the targetsubstrate. Because of the small size of these devices (e.g., smallerthan 49×40 μm²), conventional pick and place techniques are unsuitable.

SUMMARY

Embodiments relate to placing semiconductor devices such as LEDs on atarget substrate using a pick-up tool (PUT). An array of LED dies isformed including a substrate layer and a gallium semiconductor layer.The gallium semiconductor layer may be a gallium arsenide (GaAs) orgallium nitride (GaN) epitaxial layer formed (e.g., grown) on asubstrate layer, such as a sapphire, GaAs or glass substrate. Thesubstrate may be substantially transparent to a laser used for a laserlift-off (LLO) process. During the LLO process, the laser is applied(e.g., through the substrate layer) to the gallium semiconductor layerto detach the gallium semiconductor layer of the LED dies from thesubstrate layer, such that a surface of the gallium semiconductor layeris exposed and provides a light emitting surface of the LED die. Thesurface includes gallium material that results from the LLO processwhere the laser is absorbed by a portion of the gallium semiconductorlayer and converted into the gallium material. This gallium material maybe heated to melt the gallium material and form liquid gallium on thesurface of the gallium semiconductor layer. A head of a pick-up tool(PUT) is contacted with the liquid gallium, whereupon the LED die iscooled such that the liquid gallium solidifies, attaching the LED die tothe PUT. The PUT picks up and places the LED die at a desired locationon a target substrate.

The LED die can be heated to melt the gallium layer bonding the LED dieto the PUT, allowing the PUT to be detached from the LED die. Thegallium layer may then be used for subsequent pick and place operations,or be removed from the surface of the LED die. As such, the LED die canbe picked and placed using the gallium layer that is naturally formed asa consequence of the LLO process, without the need for additionalfluidic or adhesive materials for attaching to the PUT. In addition, dueto the low melting point of gallium, the gallium layer can be meltedinto a liquid and re-cooled into a solid with minimal risk of damagingthe LED die or PUT due to exposure to high temperatures.

In some embodiments, a method for picking and placing LED dies isprovided. The method comprises forming an array of light emitting diode(LED) dies including a substrate layer and a gallium semiconductorlayer. The method further comprises applying a laser to the galliumsemiconductor layer to detach the substrate layer and the galliumsemiconductor layer. The laser is absorbed by a portion of the galliumsemiconductor layer to form gallium material on each of the LED dies ofthe array. The method further comprises separating the substrate layerfrom the gallium semiconductor layer to expose a surface of the galliumsemiconductor layer and the gallium material on the surface. The galliummaterial on at least a subset of the LED dies is heated to form liquidgallium on the surface of the gallium semiconductor layer of the atleast a subset of the LED dies. The method further comprises contactingthe liquid gallium of the at least a subset of the LED dies with apick-up tool (PUT), cooling the liquid gallium into solid gallium toattach the PUT with the at least a subset of the LED dies via the solidgallium, and picking up the at least a subset of the LED dies using thePUT.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic diagram illustrating a display fabrication system,in accordance with one embodiment.

FIGS. 2A-2C shows schematic cross sections of a micro LED (μLED), inaccordance with one embodiment.

FIG. 3 illustrates a simplified schematic diagram of the μLED, inaccordance with some embodiments.

FIG. 4A illustrates a diagram of a laser lift-off operation that may beperformed to separate a μLED from a substrate layer, in accordance withsome embodiments.

FIG. 4B illustrates the μLED after it has been detached from the carriersubstrate, in accordance with some embodiments.

FIGS. 5A-5G illustrate diagrams of a process for picking and placing anμLED using metallic gallium, in accordance with some embodiments.

FIG. 6 is a flowchart of a process for picking and placing a μLED usingthe gallium layer of the μLED, in accordance with some embodiments.

The figures depict embodiments of the present disclosure for purposes ofillustration only.

DETAILED DESCRIPTION

Embodiments relate to the picking and placing of semiconductor devices,such as light emitting diodes (LEDs), using gallium material that isformed on the LEDs during the manufacturing of the LEDs. LEDs may bemanufactured by forming, among other things, a gallium semiconductorlayer on a substrate layer (e.g., a glass or sapphire substrate). Forexample, in some embodiments, red LEDs may be manufactured by growing agallium arsenide (GaAs) epitaxial layer on a substrate layer, while blueor green LEDs may be manufactured forming a gallium nitride (GaN)epitaxial layer on a substrate layer.

To manufacture a display using the LEDs, the LEDs picked up from acarrier substrate and placed onto a target substrate that forms part ofa display, or “display substrate.” A pick-up tool is used to pick one ormore LEDs on the carrier substrate and place the LEDs onto the displaysubstrate. In some embodiments, the display substrate may also bereferred to as a device substrate.

When separating the from the substrate layer (e.g., using a laserlift-off, or LLO, technique), a gallium layer may be formed on a surfaceof the gallium semiconductor layer of the LED. This gallium layer istypically undesirable and is removed prior to operation of the LEDs onthe display substrate.

In some embodiments, taking advantage of the low melting point ofgallium (about 30° C.), the gallium layer that is naturally formed onthe surface of the gallium semiconductor layer of the LED can be used toattach and detach the LED from the pick-up head of the pick-up tool fortransferring the LED to the display substrate. This may allow thepicking and placing of the LEDs without the need to apply a layer ofinterfacing material to the gallium semiconductor layer of the LEDs,thus improving the process of picking and placing the LEDs, such as fromthe carrier substrate where the LEDs are located for selective transferonto the display substrate.

System for LED Display Fabrication

FIG. 1 is a block diagram illustrating a display fabrication system 100,in accordance one embodiment. The system 100 fabricates a display deviceby assembling semiconductor devices 112 from a carrier substrate 114 toa target substrate 118. In some embodiments, the semiconductor devices112 are different color light emitting diode (LED) dies. The carriersubstrate 114 may be a carrier film that holds the semiconductor devices112 for pick up by the pick-up head array.

The target substrate 118 may be a display substrate, or may be anintermediate carrier substrate that facilitates bonding with a displaysubstrate. The system 100 places LEDs at pixel locations of the displaysubstrate, and then bonds the LEDs to the display substrate. In someembodiments, the semiconductor devices 112 are micro-LEDs having areduced divergence of light output and small light emitting area iscomparison to conventional LEDs.

The system 100 may include, among other components, a scanning electronmicroscope (SEMS) chamber 102 defining an interior environment forpicking and placing semiconductor devices 112 within the SEM chamber102. The system 100 further includes a pick-up head array 104, ascanning electron microscope (SEM) 108, an actuator 122, a carrier stage116, a target stage 120, and a laser projector 126, within the SEMchamber 102. The carrier stage 116 holds a carrier substrate 114 havingsemiconductor devices 112. The target stage 120 holds a target substrate118 to receive some or all of the semiconductor devices 112 from thecarrier substrate 114. A controller 106 is coupled to the SEM 108 andthe pick-up head array 104 (e.g., via the actuator 122) and controls theoperations of the SEM 108 and pick-up head array 104. For example, thecontroller 106 causes the pick-up head array 104 to pick up one or moresemiconductor devices 112 located on a carrier substrate 114, and placethe one or more semiconductor devices on the target substrate 118.

The pick-up head array 104 includes a plurality of pick-up heads 124.Each pick-up head 124 can pick up a semiconductor device 112 from thecarrier substrate 114, and place the semiconductor device on the targetsubstrate 118. After picking up a semiconductor device 112, the pick-uphead 124 is aligned with a location on the target substrate 118. Thepick-up head 124 is then separated from the semiconductor device 112after placing the semiconductor device 112 at the location on the targetsubstrate 118.

The actuator 122 is an electro-mechanical component that controls themovement of the pick-up head array 104 based on instructions from thecontroller 106. For example, the actuator 122 may move the pick-up headarray 104, or individual pick-up heads 124, with three degrees offreedom including up and down, left and right, and forward and back. Theactuator 122 may be embodied, for example, as a rotating motor, a linearmotor or a hydraulic cylinder.

The SEM 108 facilitates a visual alignment for semiconductor devicepick-up from the carrier substrate 114, and alignment for semiconductordevice placement on the target substrate 118. For example, the SEM 108generates images of the pick-up head array 104 and the carrier substrate114, and provides the images to the controller 106. The controller 106aligns the one or more pick-up heads 124 of the pick-up head array 104with the carrier substrate 114 based on the images, and picks up one ormore semiconductor devices 112 mounted on the carrier substrate 114. Inanother example, the SEM 108 generates images of the one or more pick-upheads 124 of the pick-up head array 104 and the target substrate 118,and provides the images to the controller 106. The controller 106 alignsthe one or more pick-up heads 124 with the display substrate 118 basedon the images, and places the semiconductor devices 112 attached to theone or more pick-up heads 124 on the display substrate 118.

In some embodiments, the SEM 108 is an environmental scanning electronmicroscope (ESEM) to provide images without specimen coating. The SEMchamber 102 is an ESEM chamber including a high pressure atmosphere ofwater vapor. The use of an SEM is advantageous for picking and placesmall semiconductor device, such as micro-LED dies. In variousembodiments, other types of imaging devices may be used to facilitatethe alignments.

In some embodiments, the carrier stage 116 and/or target stage 120 maybe adjusted to facilitate precision alignment with the pick-up headarray 104. For example, the carrier stage 116 and/or target stage 120may include three degrees of freedom. The degrees of freedom may includeleft and right, backward and forward, and a yaw rotational degree offreedom. The carrier substrate 114 is moved with the carrier stage 116,and the display substrate 118 is moved with the target stage 120.

The system 100 may include one or more carrier substrates 114. Forexample, different carrier substrates 114 may carry different color LEDdies. A carrier substrate 114 may be carrier film that holds singulatedsemiconductor devices 112 for transfer to the display substrate 118. Thesystem may include one or more target substrates 118. In someembodiments, such as when the target substrate 118 is the displaysubstrate for receiving the semiconductor devices 112, the target stage120 includes a heater for thermal conductive bonding of the electricalcontact pads of the semiconductor devices 112 to the display substrate118 subsequent to placement of the semiconductor devices 112 on thedisplay substrate 118 by the pick-up head 104. In other embodiments, thetarget substrate 118 is an intermediate carrier substrate that is usedto facilitate direct bonding of the semiconductor devices 112 with aseparate display substrate 118 (e.g., using a direct bonding process).

In some embodiments, the system 100 includes multiple pick-up headarrays 104 each positioned at a separate station. Each station may bededicated to the pick and place of a particular color LED, such as agreen station for green LEDs, a red station for red LEDs, and a bluestation for blue LEDs, etc.

As discussed above, in some embodiments, the semiconductor devices 112may correspond to microLEDs, or μLEDs. A μLED as described herein refersto a particular type of LED having a small active light emitting area(e.g., less than 2,000 μm²), and collimated light output. The collimatedlight output increases the brightness level of light emitted from thesmall active light emitting area and prevents the spreading of emittedlight into the beampath of invisible light used by light detectors andnon-visible LEDs adjacent to the μLED. While the application discussesprimarily μLEDs, it is understood that in other embodiments, thesemiconductor devices 112 may comprise any semiconductor device having agallium epitaxial layer.

Each of the semiconductor devices 112 may be formed on a substrate layer128. The substrate layer 128 may be a transparent substrate, such as aglass substrate or a sapphire substrate. In some embodiments, thesubstrate layer 128 is formed of a material that is not opticallytransparent, such as gallium arsenide (GaAs), but is substantiallytransparent to a different wavelength range (e.g., IR range). In someembodiments, the semiconductor devices 112 are formed on the substratelayer 128 by growing a gallium semiconductor layer on the substratelayer 128. Here, the gallium semiconductor layer is an epitaxial layeron the substrate layer 128. In some embodiments, the semiconductordevices 112 are placed onto the carrier substrate 114 where they areheld to facilitate detaching the substrate layer 128 from thesemiconductor devices 112.

The laser projector 126 is used to detach the substrate layer 128 fromthe semiconductor devices 112, by exposing a surface of thesemiconductor devices 112 adjacent to the substrate layer 128 to thelaser projector 126. In some embodiments, the laser projector 126 isconfigured to project a laser that is able to pass through the substratelayer 128 and be absorbed by the gallium semiconductor layer of thesemiconductor devices 112. For example, the laser projector 126 mayproject a pulse ultraviolet laser that is able to pass through asapphire substrate layer 128 to be absorbed by the gallium semiconductorlayer of the semiconductor devices 112. In other embodiments, where thesubstrate layer 128 comprises gallium arsenide, the laser projector 126projects an IR laser. Absorption of the laser beam projected by thelaser projector 126 causes a portion of the gallium semiconductor layerto separate into its component elements and weakens the bond between thegallium semiconductor layer of the semiconductor devices 112 and thesubstrate layer 128, allowing for the semiconductor devices 112 and thesubstrate layer 128 to be separated.

In some embodiments, the laser projector 126, instead of beingconfigured to project a laser through the substrate layer 128, projectsthe laser at a junction of the substrate layer 128 and the galliumsemiconductor layer of the semiconductor devices 112. As such, theprojected laser may not need to pass through the substrate layer 128. Inthese cases, the substrate layer 128 may comprise a material notsubstantially transparent to the laser projected by the laser projector126.

In some embodiments, another laser (not shown) generates a laser beam tosingulate the semiconductor devices 112 on the carrier substrate 114.Multiple semiconductor devices 112 may be fabricated on a nativesubstrate (e.g., the substrate layer 128) and singulated on the carriersubstrate 114. In some embodiments, the laser beam is directed throughthe carrier substrate 114. The carrier substrate may include a carriertape or other adhesive layer to hold the semiconductor devices 112 inplace with an adhesion force. In some embodiments, the laser beamreduces the adhesion force to facilitate pick up of the semiconductordevices 112 by the pick-up head array 104. In some embodiments, thesystem 100 includes a mechanical dicer to singulate the semiconductordevices 112, such as a diamond based cutting wheel.

In some embodiments, the controller 106, in addition to controlling analignment of the pick-up heads 124 of the pick-up head array 104 (e.g.,using actuators 122), may also control a temperature of the chamber 102.In some embodiments, the controller 106 may alter the temperature of thechamber 102 to change a temperature of the semiconductor devices 112.For example, the controller 106 may operate one or more heating coils(not shown) in the chamber 102 to raise a temperature of thesemiconductor devices 112. In other embodiments, the carrier stage 116or other component may contain a heater able to directly heat one ormore of the semiconductor devices 112. In some embodiments, thetemperature of the chamber 102 may be controller using a separatetemperature controller (not shown).

MicroLED Structure

FIGS. 2A-2C show schematic cross sections of a μLED 200, in accordancewith one embodiment. The μLED 200 is an example of a visible ornon-visible LED that may be positioned on a surface of a displaysubstrate (e.g., target substrate 118) to emit collimated visible orinvisible light.

The μLED 200 may be formed on a substrate layer 202 (which maycorrespond to the substrate layer 128 illustrated in FIG. 1), andinclude, among other components, a gallium semiconductor layer 204disposed on the substrate layer 202, a dielectric layer 214 disposed onthe gallium semiconductor layer 204, a p-contact 216 disposed on a firstportion of the dielectric layer 214, and an n-contact 218 disposed on asecond portion of the gallium semiconductor layer 204. In someembodiments, the gallium semiconductor layer 204 is grown on thesubstrate layer 202 as an epitaxial layer.

As illustrated in FIG. 2B, the substrate layer 202 may be removed fromthe surface of the gallium semiconductor layer 204 of the μLED 200 toreveal a light emitting face 210 of the μLED 200. In some embodiments,the substrate layer 202 is separated from the gallium semiconductorlayer 204 using a laser lift-off (LLO) process.

In some embodiments, the gallium semiconductor layer 204 is shaped intoa mesa 206. An active (or light emitting) layer 208 (or “active lightemitting area”) is included in the structure of the mesa 206. The mesa206 has a truncated top, on a side opposed to the light transmitting oremitting face 210 of the μLED 200. The mesa 206 also has anear-parabolic shape to form a reflective enclosure for light generatedwithin the μLED 200.

FIG. 2C illustrates the μLED 200 after removal of the substrate layer202. Upon removal of the substrate layer 202, the μLED 200 may be placedon a display substrate (not shown), and operated to emit light. Thearrows 212 show how light emitted from the active layer 208 is reflectedoff the p-contact 216 and internal walls of the mesa 206 toward thelight emitting face 210 at an angle sufficient for the light to escapethe μLED device 200 (i.e., within an angle of total internalreflection). During operation, the p-contact 216 and the n-contact 218connect the μLED 200 to a display substrate (not shown).

In some embodiments, the parabolic shaped structure of the μLED 200results in an increase in the extraction efficiency of the μLED 200 intolow illumination angles when compared to unshaped or standard LEDs. Forexample, standard LED dies generally provide an emission full width halfmaximum (FWHM) angle of 120°, which is dictated by the Lambertianreflectance from a diffuse surface. In comparison, the μLED 200 can bedesigned to provide controlled emission angle FWHM of less than standardLED dies, such as around 60°. This increased efficiency and collimatedoutput of the μLED 200 can produce light visible to the human eye withonly nano-amps of drive current.

The μLED 200 may include an active light emitting area that is less thanstandard ILEDs, such as less than 2,000 μm². The μLED 200directionalizes the light output from the active light emitting area andincreases the brightness level of the light output. The μLED 200 may beless than 20 μm in diameter with a parabolic structure (or a similarstructure) etched directly onto the LED die during the wafer processingsteps to form a quasi-collimated light beam emerging from the lightemitting face 210 of the μLED 200.

As used herein, “directionalized light” includes collimated andquasi-collimated light. For example, directionalized light may be lightthat is emitted from a light generating region of a LED and at least aportion of the emitted light is directed into a beam having a halfangle. This may increase the brightness of the LED in the direction ofthe beam of light.

A μLED 200 may include a circular cross section when cut along ahorizontal plane as shown in FIGS. 2A-2C. A μLED 200 may have aparabolic structure etched directly onto the LED die during the waferprocessing steps. The parabolic structure may comprise a light emittingregion of the μLED 200 and reflects a portion of the generated light toform the quasi-collimated light beam emitted from the light emittingface 210.

As discussed above, the substrate layer 202 may correspond to a glass orsapphire substrate. The gallium semiconductor layer 204 may include ap-doped GaN layer, an n-doped GaN layer, and the active layer 208between the p-doped and n-doped GaN layers. The active layer may includea multi-quantum well structure. The substrate layer 202 is transparentto a laser projected by the laser projector 126, which may be appliedthrough the substrate layer 202 to the gallium semiconductor layer 204.In other embodiments, the substrate layer 202 may comprise a galliumcompound, as such GaAs. The gallium semiconductor layer 204 may includea p-doped GaAn layer, an n-doped GaAs layer, and the active layer 208between the p-doped and n-doped GaAs layers. In some embodiments, theμLED 200 includes a Gallium phosphide (GaP) substrate 202 for increasedtransparency relative to GaAs, such as for red visible LEDs. In someembodiments, the substrate layer 202 is a semiconductor substrate, suchas a silicon substrate. When a non-transparent substrate layer 202 isused, the laser from the laser projector 126 may be applied at theinterface of the substrate layer 202 and the gallium semiconductor layerto separate the layers and form the gallium material to facilitate pickand place.

FIG. 3 illustrates a simplified schematic diagram of a μLED 300, inaccordance with some embodiments. The μLED 300 may correspond to theμLED 200 illustrated in FIGS. 2A-2C. The μLED 300 as illustrated in FIG.3 comprises a gallium semiconductor layer 302 having a light emittingface 310. In addition, the μLED 300 comprises a pair of electricalcontact pads 320, which may correspond to the N-contact 218 and theP-contact 216 illustrated in FIGS. 2A-2C.

Gallium Layer Formation

FIG. 4A illustrates a diagram of a laser lift-off operation that may beperformed to separate a μLED from a substrate layer, in accordance withsome embodiments. As illustrated in FIG. 4A, the μLED 400 is formed onthe substrate layer 402. The substrate layer 402 may correspond to thesubstrate layer 128 illustrated in FIG. 1 or the substrate layer 202illustrates in FIGS. 2A and 2B. In some embodiments, the substrate layer402 is a glass or a sapphire substrate layer.

In some embodiments, the gallium semiconductor layer 404 of the μLED 400comprises a gallium semiconductor compound, such as gallium nitride(GaN). In other embodiments, the gallium semiconductor layer 404 maycomprise a different gallium semiconductor compound, such as galliumarsenide (GaAs) or gallium phosphide (GaP). In some embodiments, thetype of gallium compound forming the substrate of the μLED 400 is basedupon the type of μLED. The μLED 400 may further comprises a pair ofelectrodes 406 formed on a surface of the Gallium semiconductor 404opposite from the substrate layer 402. The electrodes 406 may correspondto the electrical contact pads 320 illustrated in FIG. 3 and/or thep-contact 216 and the n-contact 218 illustrated in FIGS. 2A-2C.

A laser lift-off (LLO) procedure may be used to separate the μLED 400from the substrate layer 402, such that the μLED 400 may be picked andplaced onto a display substrate or an intermediate carrier substrate.During LLO, one or more lasers 408 are projected through the substratelayer 402 to the gallium semiconductor layer 404. The substrate layer402 is substantially transparent to the lasers 408, allowing for thelasers 408 to reach the gallium semiconductor layer 404, which absorbsat least a portion of lasers 408. In some embodiments, the lasers 408comprise a pulsed ultraviolet laser, and the substrate layer 402comprises a sapphire substrate that is substantially transparent to thepulsed ultraviolet laser. In other embodiments, the lasers 408 areprojected towards a junction of the substrate layer 402 and the galliumsemiconductor layer 404, without passing through the substrate layer402.

In some embodiments, the lasers 408 detach the gallium semiconductorlayer 404 from the substrate layer 402 by ablating the surface of thegallium semiconductor layer 404, allowing the gallium semiconductorlayer 404 to be removed from the substrate layer 402. For example, asillustrated in FIG. 4A, the GaN material at the surface of the galliumsemiconductor layer 404 adjacent to the substrate layer 402, uponexposure to the lasers 408, breaks down into its component nitrogen andgallium elements. The nitrogen may be released as nitrogen gas 410,leaving behind a layer of gallium 412. In embodiments where the galliumsemiconductor layer 404 is formed of a different gallium compound (e.g.,GaAs or GaP), different elements may be released from the galliumsemiconductor layer 404 to form the gallium layer 412. For example, inembodiments where the gallium semiconductor layer 404 comprises GaAs,the LLO process separates the GaAs at the surface of the galliumsemiconductor layer 404 into gallium metal and an arsenic compound. Thearsenic compound that is separated from the gallium metal as a result ofthe LLO process is removed to leave behind the gallium layer 412.

In some embodiments, the μLED 400 is attached to a carrier substrate(not shown) prior to being detached from the substrate layer 402. Thecarrier substrate may be attached to a surface of the μLED 400 oppositefrom the substrate layer 402 (e.g., via the electrodes 406). In someembodiments, the carrier substrate is used to hold an array of LED dies(comprising the μLED 400) to be singulated and picked and placed onto adisplay substrate or intermediate carrier substrate.

FIG. 4B illustrates the μLED 400 after it has been detached from thesubstrate layer 402. The layer of gallium 412 is formed on the surfaceof the gallium semiconductor layer 404 (comprising GaN) that wasformerly adjacent to the substrate layer 402. Typically, the layer ofgallium 412 is undesirable, and needs to be removed from the galliumsemiconductor layer 404 before the μLED 400 can be operated.

Pick and Place Operation

Once the μLED has been detached from the substrate layer, the μLED canbe placed onto a display substrate or a carrier substrate using a pickand place method. In some embodiments, a fluidic material is used toattach a head of a pick-up tool to the μLED. For example, the fluidicmaterial may form a fluidic membrane that provides a surface tension orattractive force (e.g., by covalent or Van der Wall attractive forces,or other such attractive forces) that is used to pick up the μLED usingthe pick-up tool.

In some embodiments, the existing gallium layer formed on the surface ofthe gallium semiconductor layer can be leveraged for connecting the μLEDto the head of the pick-up tool, eliminating the need for a separatefluidic material or the additional apparatuses needed for depositing aseparate fluidic material onto the μLED and/or the head of the pick-uptool. Because gallium is solid at room temperature but has a low meltingpoint (about 30° C.), the gallium layer can be melted and re-solidifiedto attach and detach the gallium semiconductor layer of the μLED to thehead of the pick-up tool, with minimal risk of damaging the μLED or thepick-up tool head.

FIGS. 5A-5G illustrate diagrams of a process for picking and placing aμLED using metallic gallium, in accordance with some embodiments. InFIG. 5A, the μLED 500 has been detached from a substrate layer (e.g., asapphire substrate) using a LLO process. The μLED 500 comprises agallium semiconductor layer 502 and a pair of electrical contacts 504.In addition, due to the LLO process separating the elements of thegallium semiconductor substrate 502, a gallium layer 506 is formed on asurface of the gallium semiconductor layer 502.

The temperature of the μLED 500 may be changed at various points overthe pick and place process to change the state of the gallium layer 506.The temperature of the μLED 500 may be controlled by a controller (e.g.,the controller 106 illustrated in FIG. 1). As illustrated in FIG. 5A,the μLED 500 may initially be at a temperature below the melting pointof gallium (e.g., <30° C.), such as room temperature, resulting in thegallium layer 506 being in a solid state.

In FIG. 5B, the temperature of the μLED 500 is raised to a point abovethe melting point of gallium (e.g., >30° C.). As such, the gallium layer506 on the μLED 500 melts to form a liquid gallium layer. In FIG. 5C, apick-up head 510 of a pick-up tool is aligned with the μLED 500, andmoved such that a surface of the pick-up head 510 contacts the liquidgallium layer 506. In FIG. 5D, the temperature of the μLED 500 islowered to below the melting point of gallium, such that the galliumlayer 506 solidifies back into a solid material. While the gallium layer506 is solid, the pick-up head 510 is bonded to the μLED 500, allowingthe pick-up head 510 to pick up and move the μLED 500.

In FIG. 5E, the pick-up head 510 picks up the μLED 500 and places theμLED 500 onto a target substrate 512. The target substrate 512 maycorrespond to the target substrate 118 illustrated in FIG. 1. In someembodiments, the target substrate 512 may be a display substrate of aμLED display device. In other embodiments, the target substrate 512 maybe an intermediate carrier substrate. During the pick and place process,the temperature of the μLED 500 may be kept below the melting point ofgallium, such that the μLED 500 remains attached to the pick-up head510. In some embodiments, the μLED 500 is bonded to the target substrate512 via the pair of electrical contacts 504. For example, where thetarget substrate 512 is a display substrate, the electrical contacts 504may be bonded to form electrical connections with the display substrate(e.g., to form a thin film transistor, or TFT layer).

In FIG. 5F, the temperature of the μLED 500 is raised to above themelting point of gallium, causing the gallium layer 506 to melt. As thegallium layer 506 liquefies, the pick-up head 510 is able to be liftedand separated from the μLED 500, leaving the μLED 500 on the targetsubstrate 512.

In some embodiments, if the target substrate 512 is an intermediatecarrier substrate, the gallium layer 506 may be kept on the μLED 500,for use in additional pick and place procedures. On the other hand, ifthe target substrate 512 is a display substrate, then the gallium layer506 may be removed in order to allow the μLED 500 to operate. FIG. 5Gillustrates the μLED 500 after placement on the target substrate 512corresponding to a display substrate. The gallium layer 506 of the μLED500 has been removed, allowing for operation of the μLED 500. In someembodiments, the gallium layer 506 is cleaned off of the μLED 500 whilestill in a liquid state. In some embodiments, the gallium layer 506 isetched off of the μLED 500 using hydrochloric acid (HCl) or another typeof etching agent.

FIG. 6 is a flowchart of a process for picking and placing a μLED usingthe gallium layer of the μLED, in accordance with some embodiments. AnLED die containing a gallium semiconductor layer is separated 602 from asubstrate layer using an LLO process, the LLO process causing theformation of a gallium layer on a surface of the LED die. In someembodiments, the substrate layer is a sapphire substrate that issubstantially transparent to an ultraviolet laser used during the LLOprocess. The laser is at least partially absorbed by the galliumsemiconductor layer of the LED die, which comprises a gallium compoundsuch as GaN or GaAs, causing a portion of the gallium semiconductorlayer to separate into its component elements (e.g., gallium metal andnitrogen gas, gallium metal and an arsenic compound, etc.) and weakeningthe bond between gallium semiconductor layer and the substrate layer. Asthe gaseous portion of the gallium semiconductor layer dissipates, agallium metal layer is left behind on the surface of the LED die. Insome embodiments, different types of laser may be used for LLO, basedupon the type of LED. For example, while an ultraviolet laser can beused for LLO for LEDs having a GaN semiconductor layer formed on asapphire substrate, an infrared (IR) laser may be used for LLO for LEDsformed on a GaAs substrate.

The LED die is heated 604 to a temperature above the melting point ofgallium (e.g., >30° C.), such that the gallium layer formed on the LEDdie enters a liquid state. In some embodiments, the LED die is heatedusing a controller that controls a temperature of a chamber that the LEDis located in. In other embodiments, heat may be directly applied to theLED die to heat the LED die to the desired temperature.

A pick-up head of a pick-up tool is positioned 606 to contact the liquidgallium layer of the LED die. The pick-up head may correspond to one ora plurality of pick-up heads of a pick-up array. In some embodiments,the pick-up head comprises a substantially flat surface having an areasimilar to that of a surface of the LED die.

The LED die is cooled 608 to a temperature below the melting point ofgallium (<30° C.), such that the gallium layer on the surface of the LEDdie solidifies and bonds the LED die to the pick-up head contacting thegallium layer. The pick-up tool picks up and places 610 the LED die ontoa desired location of a target substrate. In some embodiments, thetarget substrate is an intermediate substrate, while in otherembodiments, the target substrate may be a display substrate. In someembodiments, the electrical contacts of the LED die are bonded to thetarget substrate to form one or more electrical connections.

The LED die is heated 612 to a temperature above the melting point ofgallium, such that the gallium layer of the LED ties returns to a liquidstate, allowing for the pick-up head of the pick-up tool to be detachedfrom the LED die. In some embodiments, the gallium layer may then beremoved from the surface of LED die (e.g., if the target substrate is adisplay substrate). On the other hand, if the target substrate is not adisplay substrate, the gallium layer may be temporarily kept on thesurface of the LED die (e.g., for use in attaching to a pick-up head ofthe pick-up tool for additional pick and place operations), and onlyremoved until after the LED die has been placed on its final location ona display substrate. In some embodiments, the gallium layer on the LEDdie is removed while the gallium layer is in a liquid state. In otherembodiments, the gallium layer may be cooled such that it returns to asolid state, and removed using an etching process.

By using the existing gallium layer that is formed as a naturalconsequence of performing LLO process on an LED die to separate thesubstrate layer and gallium semiconductor layer formed on the substratelayer, the process of picking and placing the LED dies using a pick-uptool is improved in terms of manufacturing efficiency, costs, andthroughput. For example, there is no need to clean the gallium materialthat exists after the LLO process, or to attach an adhesive materialonto the LEDs dies facilitate attachment with the pick-up tool. Instead,the pick-up head of the pick-up tool can be easily attached or detachedfrom the LED die by raising and lowering the temperature of the LED die(e.g., between room temperature and above 30° C.) to change the state ofthe existing gallium layer between solid and liquid states. Because ofthe low melting point of the gallium metal, this state transition can beaccomplished without risk of damaging the LED die or the pick-up tooldue to exposure to high heat. The solidified gallium may provide astrong bond between the LED die and the pick-up head, improving theability of the pick-up tool to pick and place the LED die.

The foregoing description of the embodiments has been presented for thepurpose of illustration; it is not intended to be exhaustive or to limitthe patent rights to the precise forms disclosed. Persons skilled in therelevant art can appreciate that many modifications and variations arepossible in light of the above disclosure.

The language used in the specification has been principally selected forreadability and instructional purposes, and it may not have beenselected to delineate or circumscribe the inventive subject matter. Itis therefore intended that the scope of the patent rights be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of thepatent rights, which is set forth in the following claims.

What is claimed is:
 1. A method, comprising: forming an array of lightemitting diode (LED) dies including: a substrate layer; and a galliumsemiconductor layer formed on a surface of the substrate layer; exposingthe gallium semiconductor layer to a laser to detach the galliumsemiconductor layer from the substrate layer, the laser being absorbedby a portion of the gallium semiconductor layer to form gallium materialon each of the LED dies of the array; separating the substrate layerfrom the gallium semiconductor layer to expose a surface of the galliumsemiconductor layer and the gallium material on the surface; heating thegallium material on at least a subset of the LED dies to form liquidgallium on the surface of the gallium semiconductor layer of the atleast a subset of the LED dies; contacting the liquid gallium of the atleast a subset of the LED dies with a pick-up tool (PUT); cooling theliquid gallium into solid gallium to attach the PUT with the at least asubset of the LED dies via the solid gallium; picking up the at least asubset of the LED dies using the PUT.
 2. The method of claim 1, furthercomprising: placing the at least a subset of the LED dies on a targetsubstrate using the PUT; heating the solid gallium attaching the PUTwith the at least a subset of LED dies to detach the PUT from the atleast a subset of the LED dies.
 3. The method of claim 2, wherein thetarget substrate comprises a device substrate including control circuitsfor the LED dies.
 4. The method of claim 2, further comprising removinggallium that remains on the at least a subset of the LED dies subsequentto detaching the PUT from the at least a subset of LED dies.
 5. Themethod of claim 4, wherein removing the gallium that remains on the atleast a subset of the LED dies includes etching the gallium with anetchant.
 6. The method of claim 1, wherein the gallium semiconductorlayer includes gallium nitride (GaN).
 7. The method of claim 6, whereinthe laser being absorbed by the portion of the gallium semiconductorlayer to form gallium material separates the portion of the galliumsemiconductor layer into the gallium material and nitrogen gas.
 8. Themethod of claim 1, wherein the gallium semiconductor layer includesgallium arsenide (GaAs).
 9. The method of claim 8, wherein the laserbeing absorbed by the portion of the gallium semiconductor layer to formgallium material separates the portion of the gallium semiconductorlayer into the gallium material and an arsenic compound.
 10. The methodof claim 1, wherein exposing the gallium semiconductor layer to thelaser comprises projecting the laser through the substrate layer to thegallium semiconductor layer, wherein the laser is a pulsed ultravioletlaser, and the substrate layer comprises a sapphire substrate that issubstantially transparent to the pulsed ultraviolet laser.
 11. Themethod of claim 1, wherein the laser being absorbed by a portion of thegallium semiconductor layer weakens bonding between the galliumsemiconductor layer and the substrate layer to detach the substratelayer and the gallium semiconductor layer.
 12. The method of claim 1,further comprising: prior to exposing the gallium semiconductor layer tothe laser to detach the gallium semiconductor layer from the substratelayer, attaching the array of LEDS to a carrier substrate, the galliumsemiconductor layer being positioned between the substrate layer and thecarrier substrate; and wherein picking up the at least a subset of theLED dies using the PUT comprises picking up the at least a subset of theLED dies from the carrier substrate.